Full lines show relative phase shift, while dashed lines show scattering cross-sections versus wavelength. Inset in (a) shows the model system, consisting of a pair of dipoles oriented parallel to each other and separated by a distance L=120 nm. The particles were modelled using the coupled dipole approximation34 and size-corrected dipole polarizabilities for oblate spheroids (a=b=50(65) nm and c=10 nm) using tabulated metal permittivities41. In the monometallic cases, shown in panels (a,b), the phase drops only at wavelengths in between the plasmon resonances of the dipoles, whereas in the bimetallic dimer, (c), the phase first drops and then changes sign due to a mismatch in the permittivity between the two particles. Red and blue arrows indicate the orientation of the dipole moments induced on the particles. The green arrow indicates inversion of the dipole due to interband transitions. Circular insets in (a–c) show radiation patterns (angular resolved scattering diagrams) calculated at the antisymmetric mode frequencies (labelled inside each of the circles) and assuming particle pairs supported on an air–glass interface. The contrast varies from zero intensity dark regions to maximum intensity bright regions. Radial coordinate of the radiation pattern scales as numerical aperture NA=nsinθ and tangential as angle ϕ, with angles θ and ϕ having standard spherical coordinate meaning.

(a) An artist's view of colour routing from a bimetallic dimer supported by a glass substrate. (b) Low- and high (inset)-magnification scanning electron microscopy images of bimetallic dimers produced by hole–mask colloidal lithography. Scale bar is 1 μm (inset: 200 nm). (c) Scheme of the experimental setup. White light from a halogen lamp passes through a polarizer and is loosely focused on the sample at normal incidence. The directly transmitted light is effectively filtered out by an opaque stop in the Fourier plane, while light scattered by the dimers at angles higher than the stop (NA∼0.9) passes through to the detectors, a fibre-coupled spectrometer and a charge-coupled device camera.

Results obtained for incident polarization perpendicular (a,b) and parallel (c,d) to the dimer axis. Panels (a) and (c) summarize spectra collected in right (blue) and left (green) directions, as well as the sum in those two channels (red). Insets show corresponding direct and Fourier colour images. Directional emission is clearly observed for both polarization configurations. Panels (b) and (d) further confirm this through radiation patterns recorded at specific wavelengths corresponding to scattering to the right (450 nm) and to the left (700 nm or 600 nm) as well as through experimental directivities as a function of wavelength.

(a) Gold and silver disks (100 nm wide and 40 nm high) separated by a 20-nm gap and excited at normal incidence with light polarization perpendicular to the dimer axis. (b) The same dimer with light polarization parallel to the dimer axis. (a) The spectra correspond to light scattered within the glass hemisphere to the right (blue), left (green) and the sum the two (red). Right and left spectra are integrated over −π/2<ϕ<π/2 and π/2<ϕ<3π/2, respectively. Inset shows orientation of dimer with respect to the radiation pattern. Radial coordinate of the radiation pattern scales as numerical aperture NA=nsinθ and tangential as angle ϕ. Simulated Fourier images at specific wavelengths. The radiation pattern in (a) shows clear transitions from directional colour routing at 437 nm and 595 nm to nearly symmetric scattering at 525 and 770 nm. In (b), similar behaviour is observed at 441 nm and 561 nm, with the directivity maximum slightly shifted to blue.